A gun and bomb attack on a university in northwestern Pakistan Wednesday killed at least 20 people and wounded dozens more.

Military spokesman Asim Bajwa told reporters 18 students and two university staff were among those killed after what he described as a group of four terrorists stormed the Bacha Khan University in the town of Charsadda. Bajwa said troops, including army commandos, gunned down the four attackers and conducted a “block by block” clearing operation before securing the entire university campus.

Regional deputy inspector general of police, Saeed Wazir Khan, said at least two suicide bombers were among the attackers.

The spokesman for the Pakistani Taliban, Mohammad Khorasani, says the group has nothing to do with the university attack, adding that non-military institutions are not on its list of targets. In a statement sent to VOA, Khorasani declared Wednesday's attack against "Islamic Sharia."

Earlier, a local Taliban commander had said he sent the four attackers to the university. There was no explanation for the conflicting claims.

Students and staff at the university told the VOA correspondent at the scene, Ayesha Tanzeem, that some of the victims suffered both bullet and stab wounds.

Military spokesman Bajwa said telephone intercepts and other evidence collected from the scene have helped investigators to "swiftly achieve a breakthrough" in identifying the planners, where and how the attack was carried out. But he refused to discuss further details saying it would undermine the investigation process.

Watch: TV report from the scene. WARNING: Graphic content.

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He said the attack against a "soft target" like the Bacha Khan University shows that terrorists have been "frustrated" by successes army operations have achieved against their strongholds in the tribal areas near the Afghan border. He added that telephone calls received by the attackers on their cell phones showed they were in contact with people in Afghanistan.

Pakistani Senator Senator Shibli Faraz of Pakistan Tehreek-e-Insaf told VOA that it was alarming that militants were active after a lull in terrorists’ attacks in the country.

Reaction

The United States strongly condemned Wednesday's attack.

"It is particularly appalling that these terrorists continue to attack educational institutions, targeting Pakistan’s future generations," State Department deputy spokesman Mark Toner said in a statement. "The United States stands with the government and people of Pakistan and their efforts to create a secure, stable, and prosperous country, and we will stand side-by-side with Pakistan in its fight against terrorism."

Pakistani Prime Minister Nawaz Sharif, currently visiting Zurich, has condemned "the cowardly attack" and said he was "deeply grieved over the sad incident.

Pakistani Prime Minister Nawaz Sharif, currently visiting Zurich, has condemned "the cowardly attack" and said he was "deeply grieved over the sad incident.

An official statement quoted Sharif as reiterating Pakistan's resolve "to wipe out the menace of terrorism from our homeland."

Amnesty International said the attack "violated the central principle of international humanitarian law by deliberately targeting civilians in what appears to be a war crime."

“Whoever is responsible for this attack showed absolute contempt for life and civilian immunity. Armed groups in Pakistan must end all such affronts to humanity and commit publicly not to attack civilians,” said Champa Patel, Interim South Asia Director at the British-based group.

The university in the Khyber Pakhtunkhaw province is home to more than three thousand students.

Charsadda is located at around 50 kilometers from the province capital of Peshawar, where militants linked to the Pakistani Taliban attacked a school in December of 2014 and massacred around 150 people, mostly children.

On Tuesday, a suicide bomber blew himself up near a security checkpoint separating Peshawar from the Khyber tribal district, killing at least 12 people. The Taliban claimed responsibility for that attack.